This invention relates to the removal of solder and tin films from printed circuit boards and to a new and improved method and composition for stripping the solder and tin films and the underlying tin-copper alloy from the copper substrate of a printed circuit board in a single application of the composition as by spraying or dipping.
A typical printed circuit board uses a copper conductor attached to an insulating support, in a patterned manner. Solder is applied onto the copper substrate, typically by electroplating before the copper conductor becomes patterned into the final electrical circuit. The solder film is nominally 0.0003 inches thick which is the standard thickness used in most current day processes. When tin is used, the thickness is approximately three times greater, but the process is the same. The actual thickness may vary considerably over the board and from board to board, as in any manufacturing process. Once tin or solder is applied to the copper, a thin film of copper-tin alloy forms by solid state diffusion between the copper and solder. This copper-tin alloy film increases in thickness with time, but is typically about 0.000002 to 0.000004 inches thick.
As used in the specification and claims, the word "solder" includes the various low melting point alloys and elements used for electrical soldered connections and for copper etching masks or resists. The majority of such coatings are of various compositions of tin-lead alloys, or are substantially pure tin, but can also include lead-free alloys, pure lead coatings, and alloys containing silver, bismuth, cadmium, indium, and other metals. Such films are produced by various methods, including electroplating, chemical deposition and immersion in a melt. Some of these metals, but especially tin, can be difficult to strip effectively due to the formation of passivating films. Thus tin strips easily, if slowly, in dilute acidic stripping solutions, but may form a largely insoluble film in concentrated stripping solutions.
When applied as a bulk film, the solder may be used as a resist over the copper during etching of the copper in the production of solder mask over bare copper boards. During the manufacturing process the solder film is applied in an image-wise fashion to those areas which will be the final copper conductor. The uncoated copper is then etched from the insulating and supporting base by means of selective chemical etchants for the copper, such as ammoniacal cupric chloride solution. The solder film is later stripped from the copper substrate to allow for final fabrication, assembly, and testing. In other applications, solder is used to join two or more electrical conductors together. This solder may be removed by solder strippers for more effective rework, during salvage of board components, and during treatment of scrap and recycle boards to remove lead to reduce environmental liabilities.
Two types of compositions have been generally used in the past for solder stripping. The most widely used prior compositions were based on mixtures of an acidic solution of hydrogen peroxide and fluoride. In recent years formulations based on nitric acid solution containing ferric ion have become widely commercially used.
Acidic peroxide solutions break down during both use and storage. The stripping reaction is exothermic, thus the solution temperature increases which decomposes and wastes the unstable peroxide, while increasing both the solder etch rate and the copper etch rate. The solution requires strict control of the amount of throughput, and/or cooling during use to eliminate these problems. The fluoride content of the solutions leads to rapid attack of the fiberglass used as a component of the insulating substrate. Fluoride is both extremely toxic to the operators and difficult to effectively waste treat.
Nitric acid based solutions eliminate most of these problems. The earliest nitric acid strippers were two solution systems, consisting of a nitric acid solution to dissolve the solder, followed by a second acidic solution containing ferric ions or other materials to remove the tin-copper alloy. The second solution may contain ferric chloride, ammonium persulfate, a mixture of hydrogen peroxide and sulfuric acid, or a conventional acidic hydrogen peroxide-fluoride mixture.
The composition of U.S. Pat. No. 4,713,144 utilizes a combination of nitric, ferric and sulfamic acid which strips quickly but generates a large amount of tin rich sludge. Formulations containing organic acids such as U.S. Pat. No. 5,219,484 have been used to attempt to solve the sludge problem but were not satisfactory in that they only delayed the formation of sludge.
The composition of U.S. Pat. No. 5,244,539 utilizes a combination of nitric, ferric, and ammonium ions in combination with urea as a nitric acid fume suppressant and organic triazoles as copper anti-tarnish agents. This gives a low sludging solution but the urea reacts with nitric acid to give the heat-sensitive explosive urea nitrate which precipitates from the solution.
The basic composition and method for single bath and spray stripping is now well described in prior art patents. Commercialization of this process is dependent on meeting most of the following customer expectations: complete stripping in a reasonable minimum time, low attack rate on the exposed copper, a bright surface on the exposed copper which does not rapidly tarnish, long solution stripping life, little toxic fume evolution, and little or no sludge formation. It is an object of the present invention to provide a new and improved composition and method for solder stripping which provides fast, complete stripping without formation of a passive solder surface, with a resulting bright copper surface which tarnishes slowly, and with minimal solution fuming and sludge formation, using a single process solution.
These and other objects, advantages, features and results will more fully appear in the course of the following description.